The present invention relates to a chemical solution for forming a silver film by precipitating fine silver particles on a substrate such as glass, and in the case of production of a mirror by coating with a copper film and a corrosion-resistant resin film, forming the above-mentioned silver film finely, uniformly and with good plating efficiency and further relates to a process for forming a silver film using the chemical solution.
It is known to apply a conventional chemical solution for forming a silver film to a substrate and then allow the components of the solution to react. With this, silver is precipitated on the substrate to form a silver film. In fact, such chemical solution is a combination of (1) an ammoniac silver nitrate solution and (2) a reducing solution containing (a) a reducing agent, such as sodium gluconate, glucitol, D-glucose, tartaric acid or formaldehyde, and (b) a strong base component such as sodium hydroxide or potassium hydroxide. In the application, the ammoniac silver nitrate solution and the reducing solution are contacted on the substrate to generate the reaction.
However, in the case of simply bringing these components into contact and allowing them to react, the rate of silver plating is low, a silver film cannot be formed efficiently, and a compact, uniform silver film cannot be formed. In addition, since the resulting silver film is weakly adhered to a glass substrate and so forth, problems frequently occur in which the silver film separates and comes off of the glass substrate during edge machining and so forth of the mirror product. Moreover, in the case of immersing or allowing the resulting mirror in a corrosive solution or gas to investigate its corrosion resistance, or in the case of allowing to stand in ordinary air, the silver film is corroded in a relatively short period of time, thereby frequently resulting in the problem of it losing its function as a mirror.
It is an object of the present invention to provide a chemical solution for forming a silver film that is compact and uniform.
It is another object of the present invention to provide a process for forming such silver film using the chemical solution.
According to the present invention, there is provided a chemical solution for forming a silver film on a substrate. This chemical solution comprises (1) an ammoniac silver nitrate solution; (2) a reducing solution containing a reducing agent and a base component; and (3) an additive containing a compound of a polyvalent metal. This additive is contained in at least one of the ammoniac silver nitrate solution and the reducing solution.
According to the present invention, there is provided a process for forming a silver film on a substrate, using the chemical solution. This process comprises (a) bringing a hydrochloric acid acidified stannous chloride solution into contact with a surface of the substrate, thereby conducting a pretreatment of the surface; (b) bringing another ammoniac silver nitrate solution into contact with the surface of the substrate; and (c) bringing the ammoniac silver nitrate solution into contact with the reducing solution, on the surface of the substrate, thereby forming the silver film.
As will be clarified hereinafter, according to the present invention, effects are demonstrated that allow the formation of a compact, uniform silver film with a high silver plating rate that allows the obtaining of high-resolution reflected images, excellent adhesion to a glass substrate, and improved corrosion resistance of the silver.
The ammoniac silver nitrate solution in the chemical solution for forming a silver film refers to an aqueous solution containing therein silver nitrate and ammonium hydroxide, and is used as one of the two liquids of the chemical solution. The reducing solution of the chemical solution refers to an aqueous solution containing (1) a reducing agent, such as sodium gluconate, glucitol, D-glucose, tartaric acid or formaldehyde, and (2) a strong base component, such as sodium hydroxide or potassium hydroxide, and is used as the other liquid of the chemical solution. The chemical solution is a combination of the silver nitrate solution and the reducing solution and thus is a so-called two-package type solution. That is, the silver nitrate solution and the reducing solution of the chemical solution are brought into contact with each other on a substrate, thereby forming a silver film on the substrate. In fact, the silver nitrate solution and the reducing solution are simultaneously sprayed and mixed on a transported substrate, such as a transparent glass substrate, after which silver is precipitated by the reduction reaction resulting in the formation of a silver film on the substrate. Furthermore, a copper film is additionally formed on the silver film by a similar chemical plating during the course of mirror production, and the mirror is completed by applying a protective covering made of resin and so forth over that copper film.
In the prior art, however, since coagulated clumps of silver colloid (referred to as silver sludge) form in the solution during the course of the plating reaction, the silver plating rate decreases and a compact, uniform silver film cannot be efficiently formed as previously described. This is because, even though the silver colloid has a negative surface potential, this potential is extremely small. Moreover, since silver colloid is hydrophobic in aqueous solution, coagulated clumps form. With this, the specific surface area (surface energy) decreases, and coagulated clumps become stable. In contrast, the present invention provides the above-stated chemical solution and process that are effective in terms of uniformly and efficiently forming a silver film that has a high degree of adhesion to glass substrates and so forth, and has excellent corrosion resistance for use as a mirror product.
As stated above, at least one of the ammoniac silver nitrate solution and the reducing solution of the chemical solution contains an additive comprising a compound of a polyvalent metal, preferably having a valence of 3 or more, such as Bi(III), Al(III) or Fe(III). Thus, the additive may be this compound itself. This compound may be selected from sulfate, acetate, nitrate and other salts, chloride, bromide, fluoride and other halides, sulfide, hydroxide and so forth. Preferably, the use of the following metal compounds can be recommended.
In the present invention, examples of the metal compound containing Bi(III) include bismuth nitrate, bismuth acetate, bismuth hydroxide, bismuth carbonate, bismuth sulfate, bismuth sulfide, bismuth fluoride, bismuth chloride, bismuth bromide and bismuth iodide. These compounds form Bi(OH)3 colloid in aqueous solution. This colloid has a strong positive surface potential, and produces an electrical attraction for silver colloid having a weak negative surface potential, causing it to be adsorbed onto the surface of silver colloid. Consequently, silver colloid becomes charged with the strong positive surface potential of Bi(OH)3 colloid resulting in mutual electrical repulsion and inhibition of the formation of coagulated clumps. Examples of the metal compound containing Al(III) include aluminum sulfate, aluminum hydroxide and aluminum acetate. Examples of the metal compound containing Fe(III) include iron sulfate, iron hydroxide and iron acetate. These compounds form hydroxides having a valence of 3 or more, such as Al(OH)3 and Fe(OH)3, in aqueous solution and act in the same manner as the above-mentioned Bi(OH)3. Furthermore, since these compounds are formed over a relatively narrow pH range in aqueous solution, caution may be required when preparing the solutions. Furthermore, these metal elements can be easily detected by fluorescent X-ray analysis or wet analysis and so forth of a mirror or silver film.
The concentration of silver nitrate in the silver nitrate solution is preferably within the range of 0.01 mol/liter to 1 mol/liter. Within this range, it is more preferably about 0.1 mol/liter. The amount of the additive in the silver nitrate solution and/or the reducing solution should be determined in consideration of the amount of the above-mentioned silver nitrate, and of the ratio of the silver nitrate solution to the reducing solution and so forth. The additive is in an amount preferably of from 5 to 100 mg relative to 0.1 mol of the silver nitrate. Furthermore, The addition of an excessive amount of the additive may result in problems including decreased plating efficiency and the formation of coagulated clumps that can cause a decrease in mirror quality.
Although the silver nitrate solution and the reducing solution are normally brought into contact and reacted on the substrate by using nearly equal amounts, there are no particular restrictions on ratio of the two solutions, and the ratio of the silver nitrate solution to the reducing solution may be, for example, from (1/2):1 to 1:(1/2).
Although the reaction time of the silver nitrate solution and the reducing solution is not particularly limited, it is typically from about 20 seconds to about 40 to 50 seconds, and the reaction is essentially completed during that time. In contrast with that plating efficiency (i.e., the amount of silver plated on the substrate per amount of silver supplied) is less than 40 wt % in the case of not using the additive in a silvering test, the use of the additive may result in this becoming 40 wt % or higher.
It is preferable to employ the following procedure as a suitable process for forming a silver film on a substrate using the chemical solution for forming a silver film of the present invention. Namely, the process first goes through a preliminary treatment step in which, after first cleaning the substrate, a hydrochloric acid acidified stannous chloride solution is brought into contact with a surface of a substrate. Then, another silver nitrate solution, which may be different from that of the chemical solution, is brought into contact with the surface of the substrate. In fact, the another silver nitrate solution is free of the additive, in contrast with that of the chemical solution. In contrast with the above description, the chemical solution of the invention may be defined as further comprising the another silver nitrate. The another silver nitrate solution may contain {fraction (1/100)} to {fraction (1/10)} the total amount of silver nitrate applied for plating. In other words, the silver nitrate solution of the chemical solution may contain {fraction (9/10)} to {fraction (99/100)} the total amount of silver nitrate applied for plating. It is preferable to dilute the another silver nitrate solution to have a silver nitrate concentration of about 0.01 mol/liter, prior to the application to the substrate. After the application of the another silver nitrate solution, the above-mentioned silver nitrate solution and the reducing solution of the chemical solution are simultaneously brought into contact and allowed to react on the substrate to complete formation of the silver film.
Use of the above-mentioned process allows the formation of a more uniform silver film, and particularly in a process whereby a silver film is formed by reciprocating a spray nozzle as in the prior art. Although the occurrence of geometric film thickness unevenness accompanying movement of the spray nozzle may become a problem at the microscopic level, the above-mentioned process demonstrates actions and effects that significantly improve on this problem.
Although the following provides an explanation of the present invention by indicating several of its examples, the present invention is not limited to these examples.